A volatile memory device loses stored data when the supply of power is interrupted. Examples of a volatile memory device include a static random access memory (static RAM or SRAM), a dynamic RAM (DRAM), a synchronous DRAM (SDRAM), etc. A nonvolatile memory device retains stored data even when the supply of power is interrupted. Examples of a nonvolatile memory device include a read only memory (ROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory device, a phase-change RAM (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM or ReRAM), a ferroelectric RAM (FRAM), etc.